Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by a permeable electrolyte formed of a ceramic solid oxide, such as yttrium-stabilized zirconium (YSZ). Such a fuel cell is known in the art as a “solid oxide fuel cell” (SOFC). A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and about 1.0 volt, depending upon load, and less than about 2 watts per cm2 of cell surface. Therefore, in practice it is known to stack together, in electrical series, a plurality of cells.
In a currently-preferred arrangement, each ceramic-based fuel cell is bonded to a surrounding metal “cassette” frame to form a fuel cell sub-assembly, using a silver/copper-based braze. As the solid braze alloy is liquefied, the copper is rapidly oxidized to form copper oxide which separates from the alloy, leaving essentially pure silver as the brazing material. The copper oxide migrates to the boundaries of the liquid and adheres to the ceramic and the metal, providing an attachment layer for the silver. Exemplary silver/copper and silver/vanadium braze alloys are disclosed in International Publication No. WO 03/059843, published 24 Jul. 2003, which is incorporated herein by reference.
A problem in the use of such alloys is that the liquidus range of the alloy is very small, such that the alloy tends immediately to become very runny upon melting and is easily squeezed out of a mechanical joint, leaving a weak bond which is easily fractured. This is known in the art of bonding as a “dry joint.” Further, the alloy displaced from the joint can flow onto adjacent areas of the apparatus being brazed, such as a fuel cell assembly, which can lead to electrical shorts during later attempted use of the apparatus. In addition, if the joint gap is too large due to insufficient loading or an out-of-flatness condition of the mating parts, the braze alloy will not fill the gap.
A known approach to the problem of retaining liquid braze in a joint is to provide one or both of the surfaces to be brazed with shallow dimples such that the local joint thickness at each dimple will be increased. This approach is less than satisfactory in some applications, in that the braze layer is thickened only in the regions of the dimples, rather than over the entire joint, and in that it may be undesirable or impossible to provide the required dimples on some surfaces to be brazed.
What is needed in the art is a means for maintaining a minimum joint thickness of a brazing alloy when brazing mating planar surfaces, and for reducing the tendency of the liquefied alloy to be squeezed from such a joint.
It is a principal object of the present invention to maintain a minimum joint thickness of a brazing alloy, to reduce the tendency of the liquefied alloy to flow from a joint, and to allow an additional load to be applied to the joint to maintain a minimum gap and/or to flatten the adjoining parts without causing a dry joint.